The present invention concerns a method of injection-molding three-layer moldings, especially bottle blanks with an inner and an outer layer of polyethylene terephthalate and a middle layer of recycled polyethylene terephthalate. The method employs a mold with several cavities.
The invention is not, however, restricted to either bottle blanks or polyethylene terephthalate. It can also be employed to make moldings for any purpose and out of any plastic or combination thereof. It can for example be employed to injection mold multiple-layer blanks of polyethylene terephthalate and polyethylene naphthalate. Although both polyethylene terephthalate and polyethylene naphthalate are polyesters, the terephthalate has much better heat-resistance, mechanical, and barrier properties. It is relatively new but has already been approved by the various governmental agencies as a packaging material for the foodstuffs industry. Due to its outstanding properties, even small percentages, 10 to 25% by molding weight, definitely increase the quality of the final bottles. Bottles of polyethylene terephthalate or phthalate are particularly useful for example for bottling hot juices and for storage in hot climates.
The middle layer, made of a material called component B hereinafter, constitutes the core of the bottle wall, and the inner and outer layer, of "component A", constitute a continuous skin.
A device for carrying out such a method is known from European Patent 0 647 514 A1. The melts are supplied to the dies through a series of channels consisting of a number of bores extending through a hot-runner block. Components A and B are accordingly always at the same temperature.
The pins that close the die in this known device are pneumatically driven in accordance with the two-piston principle by two independently acting pistons accommodated in a single housing and subjected to different pressures. It is impossible to ensure synchronized opening and closing of the molten-component conveying channels that open into the various cavities and hence to obtain a uniform flow front because the compressed air responds compressibly, subsequent to a delay, that is, in accordance with the specific opening-and-closing impedances. It will not respond uniformly. The requisite synchronization of all the cavities cannot be attained, and the blanks will have different percentages of components A and B in their layers.
Another drawback of the known device is the impossibility of controlling the flow of melt or even to keep it flowing into the cavities when shifting back and forth between the components. The result is flow lines on the surface of the blank. This is because the flow of the components is also controlled by the injection pistons.